road grime

What is the best stuff to use that replicates road grime besides Tamiya weathering sets? I have Rub 'N' Buff for worn off paint on the frame.

I like to use cheap chalk pastels. I just use some sand paper and sand the pastels onto some paper in a nice little pile, then use a paint brush, cotton balls, Q-tip cotton swabs... what ever would be best for application... and go to town.

Look at your 1:1 cars for proper colors.